Atmospheric interference, also known as atmospherics, results from electromagnetic disturbances caused by electrostatic discharges between clouds, changes in ionization layers in the atmosphere due to cosmic rays, or the electrification of a receiving antenna by moving charged dust or snow particles. Another theory attributes atmospheric interference to electrons emitted by the Sun, generating eddy currents in the Heaviside layer.
The intensity of this interference varies with frequency, time of day, geographic region, and weather. The energy of atmospheric impulse interference is mainly concentrated in the frequency range below 50 megahertz. Research by Austin and others indicates that atmospheric interference strength globally increases roughly in proportion to wavelength. Above 50 MHz, interference is weaker and only minimally reflected by the ionosphere under normal conditions. At these higher frequencies, only minor local interference occurs, practically absent in HF and VHF reception.
The most significant source of atmospheric interference is lightning, occurring simultaneously in many regions of the world. On average, 2,000 thunderstorms occur at the same time, producing about 100 lightning strikes per second. Ignoring local thunderstorms, atmospheric noise levels are quasi-stationary, depending on the receiving point's coordinates and changing slowly over time and with the seasons. This predictability allows interference level forecasting, as summarized in Report No. 322 of the CCIR Plenary Assembly.
Lightning activity is highest in equatorial and tropical regions, sending most interference toward the Northern Hemisphere from the equator. Interference decreases with higher receiver frequencies and greater distances from the equator. Direction finders, preferably unidirectional, can identify interference sources. Studies revealed:
1. For North America, interference sources are in Mexico; for Western Europe – in the European part of Russia (especially in summer).
2. Daily and annual directional changes occur, often north–south in the morning and shifting eastward by noon.
3. In summer, interference usually has a clear direction.
4. Some interference has no definable direction.
At high altitudes, such as in mountains, interference levels increase. Near seas, they are lower than inland; for example, in Berlin, critical reception from America improved by locating a receiving station 400 km away on the coast. Interference intensifies in mountainous areas and rises in summer due to increased solar activity.
White noise-like interference, with nearly constant intensity across frequencies and evenly distributed spectral density, originates from Earth's electric field fluctuations, quiet discharges, auroras, and cosmic radiation. These have no preferred direction, and their sources are evenly distributed.
Various electrical processes occur constantly in the atmosphere, including cloud electrification and auroras in polar regions. The relationship between auroras and atmospheric interference is unclear. During solar eclipses, interference decreases with darkness and returns after the eclipse.
Electric currents form in ionized layers (atmospheric electricity), producing electromagnetic fields that induce alternating currents in receiving antennas. This results in crackling in headphones and loudspeakers. Antenna electrification causes continuous noise or whistling, though under normal conditions levels are low.
There is no proven direct correlation between interference and weather factors like cloud cover or humidity, except that strong interference often accompanies barometric depressions, matching the azimuth of low-pressure areas. In 1927, ships in America used direction finders to help track cyclones. Sudden weather changes typically increase interference.
A. S. Popov, the pioneering radio inventor, first identified atmospheric interference in 1895 and built the famous lightning detector in 1896. Modern studies of atmospheric discharges, using Braun tubes and oscillographs, have identified two types: a) aperiodic – average duration 3.1*10-3 seconds; b) periodic – average duration 1.9*10-3 seconds.
Related sections:
Static interference,
Industrial interference,
Signal interference,
Ionosphere, ionospheric radio propagation.